IRVING, Texas (AP) — Anthony Spencer vowed when training camp started without him that he would be at left defensive end for the Dallas Cowboys in the season opener.

Time is running short.

Spencer wasn't on the field at the start of practice Monday — six days before a visit from the New York Giants and 39 days removed from arthroscopic surgery on his left knee.

The seventh-year pro missed all of camp after an injury from offseason workouts was aggravated during the pre-camp conditioning test. Coach Jason Garrett hasn't ruled out Spencer practicing this week and isn't ready to say how much or whether he will play.

Even if he does play, Spencer will probably be limited.

"Conditioning certainly is a factor," Garrett said. "I would say particularly for Spence, just because he really hasn't done much during training camp and hasn't played in the preseason games."

Spencer is playing his second straight season under the franchise tag after leading the team in tackles and getting a career-high 11 sacks in 2012. His salary is going up to $10.6 million from $8.8 million.

The Dallas defensive front will also be without tackle Jay Ratliff for the first six games. He was placed on the physically unable to perform list with groin and hamstring issues. Nick Hayden, a sixth-year pro, is the likely starter in Ratliff's spot.

Spencer and DeMarcus Ware are the cornerstones of the pass rush in a new four-man front under defensive coordinator Monte Kiffin, but they essentially haven't been on the field together in the new scheme. The Cowboys were cautious with Ware in May and June after shoulder surgery.

Then again, their jobs are much simpler than when they were linebackers in fired defensive coordinator Rob Ryan's 3-4 defense.

"I'm not worried at all," said defensive tackle Jason Hatcher, the only starter up front who hasn't battled a significant injury at some point since last season. "Those guys have been playing long enough they can wake up in the morning and play football. Once they get on the field, they're going to be just as effective as when they left."

Spencer wasn't in the locker room when it was open to reporters Monday, but acknowledged last week it will take time to get into peak form.

"I mean, most guys won't be in top condition once they get into games," said Spencer, who is gearing up for the 4-3 alignment he played at Purdue. "When you practice, it's a whole other level than when you're in the game. Tackling somebody, getting up, getting yourself off the ground. Little stuff like that just adds to it."

The 29-year-old Spencer said he would be good for the first 10 plays, then smiled sheepishly over anything beyond that.

"I know how much desire he has to play," Garrett said. "He's a real pro, and he's doing everything he can to get healthy and be ready to go."

If Spencer can't, fourth-year journeyman George Selvie will get his first NFL start. Selvie didn't even sign with Dallas until four days into training camp after Tyrone Crawford tore his Achilles tendon in the first full practice of camp, and when the severity of Spencer's injury became clear.

Selvie was probably the best defensive player for Dallas in the preseason opener against Miami, even though rookie linebacker DeVonte Holloman returned an interception 75 yards for a touchdown.

Although Selvie leveled off considerably when he started facing first-team offenses in preseason games, he impressed coaches enough to keep the top spot on the depth chart. It's the best impression Selvie has left since St. Louis thought enough of him to play him in all 16 games as a rookie seventh-round pick out of South Florida in 2010.

"Very confident in George," Garrett said. "He's played in this league. He's a good run defender. He affects the quarterback in the pass game, and just someone you love his approach."

NOTES: G Ron Leary returned to practice Monday and could be the starter at left guard against the Giants. It would be the second-year player's NFL debut. Doug Free, who has been exclusively a tackle his first six seasons, is still getting work at guard. Free could start at right guard with Jermey Parnell in Free's old spot at right tackle. ... CB Morris Claiborne said he will play Sunday night. He didn't play in a preseason game after jamming his left knee two weeks into camp.